1) Field of the Invention
The present invention relates to a production method of foamed particles of a polyolefin resin.
2) Description of the Related Art
Foamed moldings of polyolefin resins have been widely used in various fields such as packaging materials and cushioning materials. A foamed molding of this kind is produced by filling foamed particles of a polyolefin resin, which have been produced in advance, into a mold to expand them under heat. As a production process of the foamed particles used in producing this foamed molding of the polyolefin resin, it has been known to disperse particles of a polyolefin resin and a foaming agent in a dispersion medium such as water in a closed vessel, to heat the resultant dispersion so as to impregnate the resin particles with the foaming agent, and then to release the resin particles and the dispersion medium into an atmosphere of a pressure lower than the internal pressure of the vessel, thereby expanding the particles.
Upon the production of the foamed particles of the polyolefin resin by the above-described process, the dispersing agent is generally added to the dispersion medium for the prevention of fusion bonding among the resin particles dispersed in the dispersion medium in the closed vessel. Calcium hydroxide, calcium carbonate, tricalcium phosphate, basic zinc carbonate and the like have been known as dispersing agents of this kind.
In general, the dispersing agent can not achieve a sufficient anti-fusing effect in a small amount. In addition, if it would be used in a great amount, the fusion bonding property among the resulting foamed particles upon their molding will be impeded.
On the other hand, volatile organic compounds such as flons and butane have heretofore been used as foaming agents useful in the production of foamed particles. As has already been proposed by the present applicant (assignee), it has already been known to use carbon dioxide as a foaming agent (U.S. Pat. No. 4,464,484 or EP-A-0113903). For such reasons as carbon dioxide is cheap, in recent years, carbon dioxide, mixtures of carbon dioxide and a conventional volatile organic compound such as a flon, and the like have come to be often used as foaming agents in consideration of the intended expansion ratio and the like.
The present inventors continued to investigate as to the improvement of a production method of foamed particles making use of carbon dioxide as a foaming agent on the basis of the method described in U.S. Pat. No. 4,464,484 or EP-A-0113903. As a result, it was found that when carbon dioxide and basic magnesium carbonate are used as a foaming agent and a dispersing agent, respectively, to expand resin particles, the resultant foamed particles do not undergo their mutual fusion bonding upon their production, but are well fusion-bonded to one another upon their molding (Japanese Patent Publication No. 41942/1988). In this method, the preferable amount of basic magnesium carbonate used as the dispersing agent is said to be 0.1-3.0 parts by weight per 100 parts by weight of the resin. However, a further investigation of the present inventors proved that although a good result can be obtained in this method even if the amount of basic magnesium carbonate to be used is relatively small when carbon dioxide is used in a small amount, a satisfactory result may not be always obtained in some cases if the amount of the basic magnesium carbonate to be used is within the above range when carbon dioxide is used in a large amount.
This cause is considered to be as follows. Both basic magnesium carbonate and conventional dispersing agents as described above are soluble in acids. Since a dispersion medium becomes acid when carbon dioxide is used, the dispersing agent used is dissolved in a dispersion medium, thereby reducing the effective amount as a dispersing agent. Since the acidity of the dispersion medium becomes high as the amount of carbon dioxide used increases, the effect of the dispersing agent is lowered correspondingly. It was also found that this phenomenon occurs not only upon the use of basic magnesium carbonate, but also upon the use of the above-described dispersing agents such as calcium hydroxide, calcium carbonate, calcium phosphate and basic zinc carbonate.
In order to solve such a problem, it is only necessary to add the dispersing agent in excess according to the acidity of the dispersion medium in consideration of the amount dissolved in the dispersion medium. However, this has required a complicated operation that the amount of the dispersing agent to be added is determined while taking a change in acidity of the dispersion medium, which may vary according to the proportion of carbon dioxide in the foaming agent, into consideration. In addition, there has also been a problem that since the dispersing agent in an amount more than needs is added to the dispersion medium, the cost of treatment is expensive upon waste water treatment for the dispersion medium.
According to an investigation by the present inventors, the solution of the above problem without increasing the amount of the dispersing agent to be used requires the combined use of a surfactant with the dispersing agent. However, the amount of the surfactant to be used must be increased as the amount of the dispersing agent to be used decreases. There have hence been problems that a cost is expensive due to the increased amount of the surfactant, and the cost of a waste water treatment for the dispersion medium containing the surfactant therein also comes expensive.
On the other hand, finely particulate aluminum oxide has also been known as a dispersing agent. It has been proved that when finely particulate aluminum oxide is used as a dispersing agent, the above problem can be solved even if carbon dioxide is used as a foaming agent. However, finely particulate aluminum oxide is produced only in extremely few countries. Therefore, many countries are dependent upon importation. The importing countries have involved a problem that finely particulate aluminum oxide is expensive and difficult to procure stably. There has also been a potential problem that since hydrochloric acid used in its production process remains in finely particulate aluminum oxide for all its small amount, the internal wall of a closed vessel used in the production of foamed particles may be corroded when finely particulate aluminum oxide is used as a dispersing agent over a long period of time.